Abstract
It is a characteristic of grand unified theories with an axion of the type suggested by Dine, Fischler and Srednicki that they predict a very small neutron electric dipole moment because the QCD θ vacuum parameter is suppressed. The minimal SU(5) axion model of Wise, Georgi and Glashow yields a cosmological baryon-to-photon ratio in the same order as that in the minimal SU(5) model with one 5 of Higgs, which is much smaller than required by astrophysics. More complicated grand unified axion models can avoid this problem.
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